It is well known that some types of tobacco are harvested by removing individual ripe leaves from the stalk of a standing tobacco plant to allow the remaining leaves to remain in the field to continue to grow and that other types of tobacco are harvested by removing from the field the entire tobacco stalk with the tobacco leaves still attached. The stalk is the main trunk of the tobacco plant that supports the tobacco leaves, each of which is attached to the stalk by a stem. The stem is the mid-rib of the tobacco leaf.
When tobacco is harvested by removing the entire stalk from the field, the tobacco leaves are typically cured on the stalk. After curing, tobacco farmers strip the leaves from the stalk so that the leaves can be marketed. Pieces of stalk sometimes find their way into the tobacco sold by tobacco farmers because the farmers do not always break the leaves from the stalk cleanly when they strip them. In burley tobacco production, for example, where tobacco is cured on the stalk, careless removal of the cured leaves from the stalk can result in pieces of stalk in the product.
Stalks most often end up in the tobacco product when tobacco leaves are harvested individually using mechanical harvesters for curing in bulk curing barns. Thus, it can be appreciated that although the leaves are always removed from the stalk before marketing regardless of the harvesting or curing method used, stalks frequently end up mixed in with the leaves and must be removed in a subsequent processing step.
In the threshing and separating processes employed in a tobacco stemmery, the tobacco is usually mechanically threshed and then separated onto various components. The cut tobacco forms a mixture that includes leaf parts (lamina or strip), stems, stalk parts and soil particles. As the tobacco is processed, the useful tobacco components of the mixture are separated out and the unusable portion of the mixture is discarded. When leaf tobacco is processed for use in cigarettes, chewing-tobacco or cigars, the useable end-products are the leafy particles (sometimes referred to as the "strip"), the stems, and the small leaf particles (sometimes referred to as the "fines"). The undesirable end-products are soil particles, any other non-tobacco products and the stalks. This undesirable material must be removed from the tobacco during processing.
The leaf parts and leaf particles are relatively easy to separate from the rest of the cut tobacco by, for example, exposing the threshed tobacco mixture to a vertical moving air current. Any stalks present in the mixture usually remain in the mixture with the stems after the leaf fraction is removed. However, because the stalks and stems typically have the same density, the stems must be separated from the non-usable material in a separate subsequent processing step.
Tobacco stems are used in tobacco product manufacture in one of two ways. The larger stems (greater than one and one half inches) are conditioned, rolled, cut and expanded to produce a product similar in characteristics to cut leaf tobacco. The smaller stems are ground into a pulp and mixed with the tobacco fines, water and binders to form a slurry from which reconstituted tobacco is made. In both cases, it is very important that the stems are free of non-tobacco products and stalks.
The traditional means for removing the stalks from the stems is by manually inspecting the tobacco and passing the stems over a vibrating conveyor containing a screen comprised of longitudinal rods spaced about three eighths of an inch apart. Although this method works most of the time, the screen tends to choke with the longer stems and requires frequent attention. Therefore, there exists a need in the tobacco processing business for a simpler, more efficient and more economical way to remove the stalks from processed tobacco stems.